1. Field of the Invention
This invention relates to electronic monitoring systems, and more specifically to systems for monitoring an audible signal from a rotating source to verify the operation of the source.
2. Brief Description of the Prior Art
Electronic monitoring systems are known in the art for a variety of purposes, such as, for example, the remote testing of alarm systems, including fire and burglary alarm systems. For example, U.S. Pat. No. 4,777,473 ("Weston") discloses an alarm system incorporating a dynamic range testing feature. In Weston's system an alarm condition is signaled by an open resistive element in an alarm module (e.g. caused by an intruder breaking a window to which the element is adhered or the melting of the resistive element by the heat of a fire). A central alarm system separately measures resistances in each of several modules which it polls. This system incorporates several features intended to minimize false alarm reports For example, when an alarm condition is sensed within the module itself, it is not initially reported, but instead the module must count the presence of an alarm signal in each of three consecutive rolling periods before the alarm condition is reported to the central alarm system.
Additional systems of this type are disclosed in U.S. Pat. No. 4,468,664, 4,489,312 and 4,506,255.
A second type of electronic monitoring system relates to the problem of detecting the presence of the emergency warning signal in a vehicle Emergency vehicles such as ambulances and fire trucks sound their sirens when responding to an emergency. The sirens may not be heard by the drivers of other vehicles until too late to avoid a collision for a number of reasons (e.g. the drivers may have their radios on too loud). In addition, the sound of their own siren makes it difficult to become aware of the presence of a second emergency vehicle responding to the same or a different emergency.
This second type of monitoring system is exemplified by U.S. Pat. No. 4,238,778, which discloses an electronic transmitter-receiver system for warning a motor vehicle driver of the approach of an emergency vehicle. In this system the emergency vehicle transmits a special radio frequency signal and the target vehicle must be equipped with a special receiver which generates an audible warning signal Additional patents relating to this type of monitoring system include U.S. Pat. Nos. 3,760,349, 3,949,300, and 4,443,790.
Some systems of this type equip the target vehicle with means for signaling the presence of an audible emergency signal such as a siren. For example, U.S. Pat. No. 3,867,719 employs a transducer tuned to approximately 500 hertz and mounted on the outside of the target vehicle. Means are provided to condition the signal generated by the transducer so that an indication of whether the emergency vehicle is moving towards or away from the target vehicle is given. A similar type of device is disclosed in U.S. Pat. No. 4,212,085 which provides the target vehicle driver with a visual indication of the direction of the origin of the siren. U.S. Pat. No. 3,873,963 provides a fairly simple implementation of this type of device.
Another type of audible warning system is the "civil defense" type siren, intended to provide the public at large of an imminent harzard. Although such sirens may be unmistakable to individuals in the locality where they are actually sounded, they may be remotely activated, either automatically, or well out of the hearing of an individual who activates them. To ensure the function and the reliability of these systems they must be periodically tested. There is a need for a monitoring system that which can accurately and reliably monitor when such audible waring systems have been activated, so that fault-free activation of the warning system can be verified. The system should accurately discriminate an audible signal from the warning system from the aural environment to verify the activation of the warning system for test purposes. In addition, the system should be able to operate continuously for long periods of time in the aural environment without erroneously reporting the warning system to have been activated. The monitoring system should provide for verification of correct operation of the warning system at or near the site of the audible siren, and also for verification from remote locations.